Page 14 - visitorGuide

Nature’s Processes: A Brief Lesson in Geology

Minerals are naturally occurring chemical elements or compounds of several elements. Each min-
eral species has its own definite chemical composition and many other constant characteristics, such as
color, hardness, luster, and specific gravity, which permit the mineralogist to identify it. Many minerals
form crystals in geometric shapes that reflect their internal atomic arrangement, which, again, is par-
ticular to each species.Though many of our crystal specimens appear to be carved by hand, they are all
naturally formed. No gem cutter could duplicate, time after time, the exact angles between crystal faces
that characterize every crystal of quartz, for example, wherever in the world it may have been found.

Some minerals have crystallized from molten material or have formed in other ways, but most are
formed by precipitation from solutions of various chemicals in water, hot or cold, much as crystals of
sugar form in the bottom of a syrup pitcher.

Some rocks are masses of single minerals. Limestone, for example, is made up almost entirely of
the mineral calcite. Most rocks, however, are aggregates of two or more minerals, such as granite,
which is basically an intergrowth of the minerals quartz, mica, and feldspar. Rocks are not as con-
stant in chemical composition as are minerals, and of course they possess no crystal forms.

Igneous rocks, such as granite, are formed by crystallization of minerals from molten material
deep in the earth. For example, obsidian is really a natural volcanic glass, rather than an aggregate
of minerals. It was formed from magma, or melt, similar to the kind that would have formed granite
if it had cooled very slowly. Rapid cooling of the lava from a volcano, however, caused the melt to
freeze as glass before it had time to form crystals.

Sedimentary rocks, such as sandstone and shale, are formed by accumulation of mud and sand
grains, most commonly in bodies of water. As more sediments are piled above, the lower layers be-
come consolidated and form rock. Other sedimentary deposits, such as limestone, are formed directly
by precipitation of chemical compounds from ocean or lake water.

If preexisting rocks, sedimentary or igneous, are contorted, squeezed, or otherwise changed by
mountain-building forces or chemical action, they are called metamorphic rocks. Gneiss, schist, and
serpentine are examples.

With some exceptions, rocks are usually less beautiful and attractive to the layperson than are
many mineral specimens. To geologists and petrologists, however, rocks are as fascinating and in-
structive as minerals are to mineralogists or as flowers are to botanists.

Rocks of the Rockies in the Walls

The GSA building itself is a display of specimens of most of the rocks that make up the Rocky
Mountains—the range that sits just two miles west of the building. It is reinforced concrete, poured

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